The use of bone grafts and bone substitute materials in orthopedic medicine is well known. While bone wounds can regenerate, fractures and other orthopedic injuries take a substantial time to heal, during which the bone is unable to support physiologic loads. Metal pins, screws, plates, rods, and meshes are frequently required to replace the mechanical functions of injured bone during the time of bone healing and regeneration; however, metal is significantly stiffer than bone. Use of metal implants may result in decreased bone density around the implant site due to stress shielding. Additionally, metal is less than ideal as an implant material because it remains at the healing site after healing has occurred and the need for the metal implant has passed.
The structural requirements placed upon orthopedic devices are even more pronounced when considering implants that are required to provide structural support to a human spine. Spinal fusions require interbody fusion devices that will maintain significant structural rigidity for at least 6-12 months, and strength requirements depend on the location of the disc to be replaced. When a person is standing, the forces to which a disc is subjected are much greater than the weight of the portion of the body above it. It has been reported that the force on a lumbar disc in a sitting position is more than three times the weight of the trunk.
During the course of a bone's cellular healing processes, through coordinated activity of osteoblast and osteoclast cells, bone grafts and certain bone substitute materials placed at the site of an injury can be removed by natural processes over time and replaced by endogenous bone that is almost indistinguishable from the original. An intact bone section harvested from a human donor can be formed into a monolithic bone graft in some cases; however, the use of such bone grafts is limited by the available shape and size of grafts and the desire to optimize both mechanical strength and replacement rate relative to the timeframe of fracture or defect healing at the skeletal site. Variations in bone size and shape among patients (and donors) also make monolithic bone grafts a less optimal substitute material.
Some bone substitute materials and bone chips can be used to form grafts of desired shape, and are quickly degraded, but such materials cannot immediately provide mechanical support. Cancellous bone allografts have open spaces for easy cellular penetration and biodegradation, but they lack appropriate initial strength for many load bearing applications. Cortical bone grafts are stronger than cancellous grafts but are more slowly and incompletely replaced by endogenous tissue. While the extent of integration of cortical bone grafts is generally considered adequate, it has been reported that endogenous replacement of such a graft seldom exceeds more than 50%. For these reasons, significant attention has been given in recent years to the development of orthopedic implant materials formed from polymeric materials that have mechanical properties approximating those of bone, i.e., that are suitable for load bearing, and that undergo extensive transformation into native tissue at a desirable rate.
While a number of polymeric materials have been developed and used for making implant composites, significant obstacles have been encountered. Traditionally, the processing of such polymers has been achieved via melt processing at temperatures high enough that the polymer is melted and can flow (typically 150 to 300° C.) or through solvent-aided processing in which the polymer is dissolved in a solvent, then molded and dried. A problem that has been encountered with the melt processing approach is that, though it is desired to incorporate bioactive materials, such as, for example, tissue-derived materials, into the implant, functionality (bioactivity) of many such materials is often compromised at the high processing temperatures of such protocols, and thus cannot be present during high-temperature processing of the polymeric phase. A problem with solvent-based processes is that a relatively long period of time is required to remove the solvent from the mixture. In addition, some solvents that would otherwise be useful to dissolve the polymers can compromise the functionality of bioactive materials.
With the development of a wide range of spinal prosthetic devices, and the use of a wide range of polymeric materials and bioactive agents to manufacture the same, there is a growing need for better ways to manufacture such devices. The present invention addresses this need.